Brake fluid question
 

I have had a phase 2 (X-reg) 1.9 diesel V40 for about a year now. Today the brake warning light failed to go out. When I checked, the fluid was just level with a little tube that leads off from the fluid reservoir. The brakes were working as normal. So I took the lid off and topped it up with dot 4 as specified. But inside the lid is a black bulbous thing, made of thin soft plastic. When I put the lid back on, a lot of the brake fluid came pouring out again. There's no mention of this in the Haynes manual. I can remember having this problem some time last year. If the level where the light stays on is 'MIN' where should 'MAX' be? It didn't take a lot of fluid. And what's the big black thing for? (Sorry if this is a common problem that everyone else knows about.)

Normal, just press the black bit back into the lid, it should collapse and fit into it nicely.

I'd be concerned about where your fluid is going.

Thanks for that. I will try pressing it back in. I assumed that the fluid went down as the brake pads wore? Perhaps if I had filled it up properly the first time it would not have been low this time. The first time, I just topped it up as part of regular maintenance, not because it was low.

The black insert in the cap follows the brake fluid as the level falls and needs pushing back into the cap when refilling with fluid. It is designed to eliminate as much air as possible in the reservoir thus preventing moisture contaminating the fluid. One of the little niceties that Volvo think of.

Have you checked the condition of your pads and discs? As when the pads wear the fluid level drops in the reservoir as it's in the pipes because the piston is further out.

Yeah, our car has the black bulbous thing, and it is frustrating. I too have had the fluid spill out, exactly as you describe. The only solution I've found is to be aware of it and try to judge the quantity appropriately, and it takes practice, but it's not something you will do very often.

I didn't know about it "following the fluid down", although it sounds plausible. Mine doesn't do this, it simply displaces any fluid in it's way. It's soft enough to squash with finger pressure, but not with fluid during screw-down, it just spills.

As said above, if you're needing to top it up - ever - it's because you have a leak. That needs fixing.

Get a hose pipe on the area, drench the thing in water, and don't be prudent with it. Brake fluid will destroy the paint on the subframe, which it almost certainly where it ended up, and therefore rust. And the steering rack, and it's pipes and bolts, and bodywork. That ain't cheap to fix.

Yes, I had to pour boiling water over the cap to get it to move at all. So I used the rest of the (very hot) water to wash it down after it spilled. Today I have pressed the soft thing in, but it isn't neat. I can now see the 'MAX' arrow, well concealed, and that is where the fluid comes to after being spilled out. I will go to the garage if the level falls again. Thanks to all for your input.

The concept is missed completely.

The fluid is topped up to max and the black diaphragm is manually pushed back into the cap. Cap screwed on the reservoir and all is well with no fluid being spilled. As the pads wear the fluid in the reservoir drops and that in turn pulls the diaphragm down so that it follows the fluid level and excludes air from the top surface of the fluid, minimising moisture contaminating the fluid. When next topping up the fluid or winding back the pistons the black diaphragm needs pushing back in the cap before refitting or else fluid will be spilt.

Still a bit confused. You press in the diaphragm, and go to put it back on the reservoir, it pops out again just in time for you to reach the fluid where it hits the surface and displaces a load all over the sides. I'm not getting this, am I?

ah is it worth checking for any minor leaks eg from flxible connectors ?